This invention relates generally to connectors for mounting circuit elements such as large scale integrated (LSI) chips on a substrate and more particularly to mounting LSI chips on a substrate wherein the thermal coefficient of expansion of the connector and the substrate can be different.
One of the more extensively used means for mounting integrated circuit chips on a substrate has been by means of a dual-in-line (DIP) package having a plastic housing constructed to hermetically retain the LSI chip. The LSI chip can be mounted upon first ends of a pattern of conductive leads known in the art as a spider lead assembly, the other ends of which are secured to first ends of the conductors of a lead frame structure. The lead frame conductors extend through the plastic housing and can be secured in apertures formed in a printed circuit board. Ordinarily such apertures are plated-through holes in the printed circuit board but sometimes they consist of female receptacles or sockets mounted on, or inserted in, the printed circuit board and designed to receive the terminal pins of the DIP connector.
For several reasons, including among others, the cost of manufacturing DIP receptacles, packaging space considerations, reliability of the product, and problems of inserting the fragile DIP terminals into apertures, considerable effort has been expended to develope a connector which will retain an LSI chip and which has external terminals which can be secured directly to the surface of a substrate, as by soldering, for example.
An early type of such connector employs a ceramic encapsulation, generally square in shape and multilayered, which contains an LSI chip with leads extending from the LSI chip through, and external to, the ceramic housing. Such external terminals, which are relatively rigid and configured somewhat in the nature of conductive pads, are secured to correspondingly positioned conductive pads on the surface of a ceramic mother board or hybrid substrate, as by soldering for example.
With such an arrangement, particularly in the larger chip carriers with leads of from 24 on up to 64, there is a considerable space advantage over DIP structures having an equal number of leads. More specifically, the space requirements on the substrate when the chip carriers are connected directly thereto is approximately one third that required by conventional DIP packaging structures. Further, the use of a chip carrier, as opposed to a DIP, results in a considerably reduced circuit path length because of the absence of the longer DIP terminals. As a result of the reduced circuit path lengths the lead inductance is also substantially reduced, thereby permitting operating frequencies up to three times the frequency which can be employed with a DIP package.
While a ceramic package mounted directly on the substrate has the advantages set forth above, the use of a ceramic substrate has several disadvantages. For a number of reasons, plastic printed circuit boards such as G-10 epoxy, currently are extensively employed in lieu of ceramic substrates. However, the G-10 boards have a thermal coefficient of expansion which is much greater than that of the ceramics employed in chip carriers. Accordingly, soldering of the contact pads on a ceramic chip onto the contact pads on the surface of the plastic printed circuit board would easily crack or break with any substantial change in temperature. Even the use of a plastic chip carrier having external contact pads which are soldered to contact pads on a printed circuit board having a similar temperature thermal coefficient of expansion presents certain problems.
More specifically, while the thermal coefficients of expansion of the plastic chip carrier and the printed circuit board might be similar in the X-Y coordinates, many printed circuit board designs, particularly double sided boards, have stresses produced on the opposite sides of the board due to the difference in orientation of the circuit paths provided thereon, which can produce bow in the board. Such bowing or deflection of the board in the Z axis can break the solder joints even when both the chip carrier and the circuit board are of plastic having a similar thermal coefficient of expansion.
In certain applications, it is desirable that the LSI chip carrier, or other circuit element, be removably connected to the surface of a printed circuit board. Such a removable connection can be implemented by providing a two part connector with the first part being the LSI chip carrier, which can now be either plastic or ceramic, and the second part being an interface between the first connector part and the circuit board. The second part can be permanently secured to the circuit board and the first part removably mountable in the second part. However, the permanent connection of the second part upon the circuit board presents the same problems of different thermal expansions as exist when the first part of the connector, i.e., the LSI chip carrier, is connected directly to the surface of the printed circuit board.